Electrohydraulic power-assisted steering systems of the generic class are well known. A power-assisted steering valve is directly or indirectly coupled with a vehicle steering wheel and put under hydraulic pressure. On torsion, the hydraulic pressure is guided to one or the other side of a steering gear and provides steering power assistance. The hydraulic pressure is provided by a hydraulic pump, which in turn is driven by an electric motor.
Typically, the electric motor runs at a predefined speed, which in turn ensures a corresponding speed of the hydraulic pump and thus provides a desirable operating pressure. As a function of the steering rate and the steering torque, a hydraulic flow under operating pressure is produced, which effects the described power assistance. The obtained operating pressure is in direct proportion to the current input to the electric motor.
In such power-assisted steering systems it is desirable in principle that the pressure should not significantly exceed the actual maximum operating pressure. This makes it possible to design all valves, connections, lines and the like of the power-assisted steering system for the respective pressure and thus ensures the cost-effectiveness of the power-assisted steering system as a whole due to the use of economically selected components. An uncontrolled increase in operating pressure would make it necessary to select overdimensioned individual elements so that the power-assisted steering system could no longer be economically produced.
Prior art limits the pressure in upward direction by using so-called pressure control valves. These are valves opening under hydraulic pressure, which open a bypass and thus prevent overpressure on other components of the power-assisted steering system.
It is known that the closer the opening range of the pressure control valve to the maximum operating pressure and the more precise the opening of the pressure control valve, the higher the corresponding production costs. Pressure control valves, which respond when the maximum operating pressure is exceeded by approximately 5%, are relatively large and costly components but are very stable in operation.
In contrast, another type of pressure control valve, whose opening range is approximately 20% above the desired maximum operating pressure, has become generally accepted in practice so that the valves are smaller and less expensive to produce. A disadvantage of these valves is that they are stable only if the valve characteristic is flat. These pressure control valves, which are not as sensitive as the first mentioned valves, require, however, that the other elements of the power-assisted steering system be dimensioned accordingly.